In radio communication, echo signals can occur over a radio channel as a result of multipath propagation of a transmitted radio signal. The transmitted signal reaches a receiver both directly and also via one or more reflected or otherwise deflected signal paths. In the case of digital transmission systems, the echo signals give rise to intersymbol interference. The problems resulting from this interference are well known and solutions to the problems are described in the literature, for instance in an article in IEEE TRANSACTIONS ON INFORMATION THEORY, Vol. IT-18, No. 3, May 1972, G. D. Forney: "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference". The receiver has an adjustable filter which is set with the aid of a known synchronizing sequence. The filter is an image of a sampled impulse response for the channel, usually designated a channel estimate, whose parameters are used to establish the values of transmitted symbols. If the channel is changed with time, the channel estimate is adapted, possibly with the aid of the established symbols, for instance as described in an article in IEEE TRANSACTIONS ON INFORMATION THEORY, January 1973, pages 120-124, F. R. Magee and J. G. Proakis: "Adaptive Maximum-Likelihood Sequence Estimation for Digital Signaling in the Presence of Intersymbol Interference". In the case of channels which are changed quickly in relation to the transmitted bit frequency, further problems arise because the adaptation process must be effected so quickly that it generates noise itself. The channel estimation herewith becomes sensitive to errorneous decisions. Swedish Patent Application No. 8903526-5 describes an equalizer which overcomes these problems. The equalizer is provided with an analyzer which operates in accordance with a Viterbi algorithm having a requisite number of states. Each state is assigned a channel estimate, which is adapted in accordance with selected state transitions in the Viterbi algorithm. This adaptation is effected without time delay in the analyzer. The problem of erroneous decisions during adaptation of the channel estimation is particularly apparent in the case of fading, where the signal strength rapidly decreases and fades away and thereafter rapidly increases. The fading phenomenon is caused by interfering radio signals and is described in detail by William C. Y. Lee in Mobile Communications Engineering, Chapters 6 and 7, McGraw-Hill, Inc., 1982. In order to overcome the estimation problems caused by fading, there is required a better model of the channel than that obtained with the aforesaid methods. One proposal for such improved channel estimation is given in IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. 37, No. 9, September 1989, A. P. Clark and S. Hariharan: "Adaptive Channel Estimation for an HF Radio Link". This article suggests generally the use of the channel estimate derivative in channel estimation processes. The article, however, gives no indication as to how the derivative shall be initiated, for instance, during the estimation process so that a reliable derivative will be obtained. When a plurality of symbols are lost, particularly with fading it is essential that such a reliable derivative is obtained.